The present invention generally relates to release surfaces of the type used with pressure-sensitive adhesive (PSA) constructions. More particularly, the present invention relates to multilayer release liners and their methods of manufacture.
A pressure-sensitive adhesive construction, such as a label, generally comprises a facestock or label surface, an adhesive composition adhered to the facestock, and a release liner. The adhesive composition is typically coated on a silicone-containing release surface of the liner. Alternately, the PSA can directly be coated onto the facestock and then be laminated to the release liner. In these combinations, the release liner protects the PSA prior to the label being used and is removed immediately prior to application of the label to another surface. Additionally, the release liner serves to facilitate cost effective manufacture of rolls or sheets of labels. The release liner also functions as a carrier of labels for dispensing in automatic labeling operations and for computer printing in EDP applications. The performance attributes of a release liner are critical to both the manufacture and end-use application of adhesive labels.
In conventional practice, the release liner is provided as a silicone layer on a paper or film surface having high holdout, i.e., the surface of the paper on which the silicone layer is deposited is resistant to silicone penetration. This is preferred because silicone tends to be an expensive component of a release liner, and it is therefore desirable to minimize the amount of silicone coated. High quality papers conventionally used in the manufacture of release liners, such as a super-calendered or densified glossy paper, achieve this goal by providing a surface which absorbs much less silicone than regular open paper. However, use of these high quality papers increases the cost of the end product adhesive construction, because such papers are typically much more expensive than regular open papers.
One currently accepted way of applying a silicone release composition to a high holdout paper is by solvent coating. Growing concern over the environment has imposed stringent restrictions regarding recovery of the solvent used in applying the solvent based silicone to the high-holdout backing paper or other materials. An alternative to this is to use 100% solids silicone release compositions. These are supplied with a viscosity (usually  less than 2000 cps) suitable for roll-coating techniques. When applied to porous low cost papers, such as machine finished (MF) or machine glazed (MG) papers, these materials soak into the paper (penetrate the paper surface) to give ineffective coverage of the paper fibers unless excessively high quantities of expensive silicone are used. Ineffective coverage of the paper fibers provides unsuitable release liners for PSA applications, especially where high speed convertibility is an essential performance feature.
One proposed prior art solution to these problems is to use low cost open papers which have been pre-coated with a support layer comprising an inexpensive filler material, and then to coat the silicone layer onto the support layer. The filler material of the support layer flows into the pores and interstices of the open paper surface which would otherwise absorb silicone if the silicone were directly coated onto the paper. Consequently, less silicone is needed to form an adequate release surface. An example of this approach may be found in U.S. Pat. No. 4,859,511 to Patterson. However, there are several drawbacks to this prior art process. First, additional costs are incurred because the prior art methods require two coating steps. The prior art teaches that the support layer must first be coated and then dried, cured or hardened before the silicone layer may be coated. Otherwise, there is a potential for undesirable intermixing or destruction of the respective layers. Second, because two separate coating steps are required, more time is needed for the overall formation of the release liner. These additional processing costs somewhat offset the savings realized in materials by using support layers in combination with lower cost open papers.
Thus, there is a need for improved methods of forming multilayer release surfaces in which a support layer is used in conjunction with a silicone layer to form a release liner.
The present invention advantageously provides an efficient method of creating multilayer release liners, thereby overcoming the problems resulting from the prior art processes. The present invention achieves these benefits by providing a method of coating both a support layer and a release layer on a substrate substantially simultaneously. Consequently, separate coating steps are eliminated, and a corresponding savings in both time and costs are achieved.
Generally, these advantageous results may be achieved by at least two different coating methods. The first method achieves these results by modifying the die used to coat the support layer and a release layer (e.g., silicone layer) so that the die can dispense the fluids of both layers substantially simultaneously at a single coating station. The die dispenses the support layer onto the substrate, and substantially simultaneously, the release layer on the support layer. There is no need for a separate drying, hardening or curing step to prevent the layers from intermixing. By controlling the coating gap between the die and substrate, the processing conditions of the modified die may be optimized to achieve the most stable and efficient deposition of these layers. In some embodiments, application of controlled vacuum to the dual die coating process may be used to improve coating efficiency, increase coating tolerances and provide for less penetration of coated fluids onto the substrate to be coated. The multilayer release surfaces resulting from the simultaneous dual die coating of support layer and silicone-containing layer are also believed to have a unique morphology and advantageous properties.
Simultaneous coating of the support and release layers to form a multilayer release surface may also be achieved by curtain coating. For example, a slide coat die may be modified to have two slots, with the upper slot metering the release layer and the lower slot the support layer. The release layer and support layer combine on the die face surface, and fall to the moving substrate as a multilayer liquid sheet. The distance between the die and the substrate may range from 5 cm to 50 cm, and more preferably, from 10 cm to 30 cm. Advantageously, curtain coating techniques do not require as precise an optimization of the coating gap between the die and the substrate to generate high speed coatings, and high coating speeds are easily obtained.
With respect to simultaneous coatings using a dual die, the present invention provides a method that is capable, at steady state coating conditions, of precisely controlling the interface or xe2x80x9cseparating streamlinexe2x80x9d between the support layer and silicone-containing layer as these layers are being coated onto the substrate. Unlike single-layer coating, the stability of the flow (i.e., its tendency to exhibit only a steady, two-dimensional flow) particularly at the separating streamline between the two layers, is extremely important. Advantageously, this method can be used to substantially simultaneously coat a support layer on a backing in conjunction with a silicone-containing release layer on the support layer. As used herein, substantially simultaneously refers to two or more liquid layers being deposited at a single coating station without an intermediate drying, curing, or hardening step for the support layer. For die coating, preferably, the single coating station comprises the dual die described herein, although this is not essential to the present invention. For example, the single coating station may comprise two separate dies located close enough spatially to achieve the benefits of a dual die.
The present dual die method involves a number of preliminary steps, the sequence of which is not particularly important. These steps include an analysis of certain parameters of the liquids to be coated, the particular and precise design of the geometries of the die lips, and the assembly or setup of the die with respect to the moving web. Following these steps, a number of experimental release-surface coatings can be made in order to determine an operating window for achieving successful multilayer dual die coating. Even within this window, a higher quality window can be determined for full production coating operation. These steps assist in providing a stable, two-dimensional flow.
An unstable flow changes its profile with respect to time. This can result in random fluctuations or regular oscillations in the flow profile, thus causing irregularities in the cross-sectional film configuration. In addition, slight perturbations in the coating process under unstable conditions may propagate, rather than dampen out quickly to a steady state condition as with stable flow. Likewise, a three-dimensional flow may result in undesirable mixing of the two layers, or in cross-web, nonuniform layer thickness, as well as other defects such as non-continuous layers or voids, etc. In stable, two-dimensional flow each layer has greater uniformity, thus resulting in a product of higher integrity and performance. Furthermore, if the flow is perturbed, this type of flow will return to its steady, two-dimensional flow characteristics rapidly, thus minimizing any defects in the product.
The coating method of the dual die aspect of the present invention achieves a stable, two-dimensional flow by controlling the interface of the flow at its upstream most position, which is referred to herein as the separating streamline or separating line. This line is defined, in the sense of web travel, as the cross-web line where the topmost streamline of the bottom flow layer (i.e., support layer) first meets the bottommost streamline of the top flow layer (i.e., silicone-containing release layer). In the opposite direction, the separating line can be viewed as the location where the two flows separate from the die lips. Although the separating line runs completely across the web, when the die/web interface is shown from the side, it appears as a point. As noted, this separating line will occur in the region of the mouth of the downstream slot or feed gap where the flows of the bottom layer and top layer are confluent. For ease of reference, this region will be referred to herein as the xe2x80x9cinterface region.xe2x80x9d It will be understood that if the combined flow of the two layers is stable and two-dimensional in this interface region, and more particularly at the separating line, it is likely to retain such flow characteristics throughout the coating process, thus resulting in an improved end product.
In order to achieve such advantageous flow characteristics at the separating line, the multilayer coating method of the present invention assists in positioning that line at the downstream corner of a die middle lip. This corner presents a straight, two-dimensional line across the die. Thus, if the separating line is coincident at this corner, one will be assured of achieving stable, two-dimensional flow. For this reason, this corner is referred to herein as the xe2x80x9cstability point.xe2x80x9d On the other hand, it will be appreciated that unstable or three-dimensional flow conditions can cause the separating line to occur at several locations in the interface region. For example, xe2x80x9crecirculationsxe2x80x9d in the bottom layer flow can cause the top layer flow to be pulled upstream such that it separates from a position underneath the middle lip. Likewise, vortices or other stagnant flow in the top layer can cause the top layer to separate from the middle lip at a position within the feed gap of that flow.
Stable, two-dimensional flow characteristics in the dual die interface region are achieved in the present invention due in part to a method of regulating the pressure gradient such that the separating line is positioned at the stability point. In accordance with one method of the present invention, the pressure gradient can be regulated by designing and assembling a die having a particular middle lip geometry. This method of pressure regulation helps to pin or lock the separating line at the stability point. This is achieved, as the name implies, by regulating the pressure gradient in the interface region. As is well understood, the pressure gradient in this region is highly dependent on the coating gap and its relationship to the downstream film thickness. In accordance with complex but well understood principles of fluid mechanics, the pressure gradient created at a particular longitudinal portion in the bead is related to the coating gap at that point and the downstream thickness of that flow. Here, however, much care must be taken in the analysis. Indeed, for a single-layer coating the analysis is more direct, since there is only one flow, and one downstream film thickness. However, for a multilayer coating process, there are two or more flows. Thus, in a method for regulating the pressure gradient at a given point in the flow, the coating gap at that point and the downstream film thickness of the layer(s) formed by that flow must be analyzed in order to achieve proper lip design and positioning parameters.
Therefore, an analysis of the pressure gradient within a particular flow, and particularly the pressure gradient of the combined flow at the interface region, is quite complex.
The dual die method of the present invention designs the middle and downstream die lip geometries such that the pressure gradients in the flow fix the separating line at the stability point. In another aspect of the method, the middle lip may extend slightly toward the web. Therefore, the profile formed by the design of the middle and downstream lips of the die represent a step away from the web in the direction of web travel. This step configuration may be flat or parallel with respect to the web or angled with respect thereto. It may even exhibit other designs. It is only important that certain pressure gradients be maintained in the interface region, and particularly along the middle coating gap from the stability point toward the upstream corner of the middle lip.
It has been observed that the magnitude of the step may be in the range of 0 to about 100 microns inches when coating multilayer adhesive compositions. However, for dual die coating of a support layer and a silicone-containing layer, it is preferred to minimize or eliminate the step. Consequently, for these multilayer release systems, it is presently preferred that any step be in the range of from 0 to about 50 microns, with the optimal step approaching zero. Minimizing or eliminating the step in this manner optimizes the multilayer coating process for silicone release system.
When a stepped design is used, it should be appreciated that the die lips stepped design affects the coating gap under both the middle and downstream lips in the interface region. Because the middle lip is stepped toward the web, the coating gap under this lip will be less than that under the downstream lip. As a result, for most multilayer coatings, if the die is correctly positioned with respect to the web, the pressure gradient under the middle lip will be very slightly positive to approximately zero, while the pressure gradient under the downstream lip will be negative. When these stepped dies are used with these pressure gradient differentials, the gap under the middle lip can be from two to three times the film thickness, with the corresponding pressure gradient under the middle lip again being from slightly positive to zero. Again, this relationship exists at least in the interface region close to the mouth of the downstream feed gap. Due to other lip designs (such as bevels) and adjustments in the angle of attack of the die, the relationship between the pressure gradients under the middle lip and under the downstream lip may vary differently. However, in the interface region it is important that the pressure gradient at or just upstream of that region not be excessively positive in the direction of web travel.
If the pressure gradient is too high in this region, certain instabilities in the flow may occur, resulting in coating defects. For example, in the absence of proper pressure gradient regulation, the bottom layer flow may exhibit xe2x80x9crecirculationxe2x80x9d under the middle lip. This could occur, for example, if the downward step in the middle lip is not properly adjusted, and an excessively large coating gap occurs in this region. Desirable pressure gradients may be achieved for dual die coating multilayer silicone release systems when the step of the middle and downstream lips is minimized. Furthermore, the coating gap of the middle and downstream lips may be from 2 to 3 times the total wet film thickness. A larger coating gap results in a highly positive pressure gradient in the bottom layer flow, causing it to actually flow upstream a short distance before turning around and flowing downstream, causing xe2x80x9crecirculationxe2x80x9d of the flow. One of the most serious disadvantages of such recirculations in the bottom layer flow is its tendency to pull the top layer flow upstream under the middle lip and away from the stability point. Thus, the separating line moves upstream and there is no assurance that the line will be formed in a straight and steady manner. Thus, mixing and diffusion between the two layers at their interface may increase. In addition, the flow may be mottled or blotchy. Other defects can be caused by recirculations. Recirculations are of two types: open loop and closed loop. Open-loop recirculations are less damaging because any liquid entering them leaves after a short period of time (low xe2x80x9cresidence timexe2x80x9d), before continuing to flow downstream. Closed-loop recirculations, however, result in high residence time because the liquid is trapped in them. Moreover, all recirculations are known to prefer three-dimensional flow characteristics.
On the other hand, the pressure gradient under the middle lip cannot be too large (which might occur, for example, if the coating gap in this region were too small). Such a large pressure gradient is likely to result in upstream leakage of the fluid. Also, as mentioned above, such high pressure gradients can result in high shear stresses with other deleterious effects on the performance of the coating.
It will also be observed that the step designed into the middle lip can be achieved by positioning that lip at the proper coating gap and moving the downstream lip further away from the web. However, there is also a tradeoff in this parameter. If the coating gap under the downstream lip then becomes too large, recirculations or vortices in the top layer flow may result. One additional type of defect that may occur is known as xe2x80x9cchatterxe2x80x9d, or a two-dimensional oscillation of the bead.
Thus, an important advantage of the method of the dual die aspects of present invention is that it provides a proper pressure gradient ahead of the interface region for the coating of multilayer silicone systems. However, as explained, this advantage can only be achieved when the die is correctly set with respect to the web in order to exhibit proper coating gap characteristics. Preferably, it has been found that the die should be set such that the coating gap under the middle lip (especially in the interface region) is approximately two to three times the bottom layer wet film thickness downstream of the die (before drying). It should be re-emphasized that this thickness, however, is the thickness of the bottom layer only which is being coated from this particular flow under the middle layer. Similarly, the coating gap under the downstream lip (particularly in the interface region) should be greater than one but not greater than three times the wet film thickness downstream, to provide the least pressure under the lips and therefore minimize flow of material into the paper. In this latter case, this thickness is the combined thickness of both layers as well as any previous layers. Thus, it will be understood that these principles apply to multilayer coating of any number of layers, with the terms xe2x80x9cbottom layerxe2x80x9d and xe2x80x9ctop layerxe2x80x9d referring to any two adjacent layers. It will also be recognized that these relationships will slightly vary due to non-Newtonian characteristics of the liquid, as well as other variables.
On the other hand, the method of the present invention allows for optimization of the dual die multilayer coating process. In one aspect of the method, the middle and downstream lips are flat or parallel with respect to each other. Thus, any convergence of the downstream lip can be achieved by adjusting the angle of attack of the die. In another aspect of the method, however, the optimization of the coating process is facilitated by beveling the downstream lip so that it exhibits some convergence, even without any angle of attack adjustment. With this improvement the xe2x80x9coperating windowxe2x80x9d of the die can be increased. This means that successful coating can be achieved, even if certain coating parameters cannot be accurately controlled. On the other hand, a larger operating window increases the chance of a larger quality window where the best coating occurs. Moreover, a large operating window allows a technician of less skill or experience to successfully perform the coating operation. In addition, a wider variety of products comprised of a broader range of liquids can be produced, even single-layer products.
In another aspect of the present invention, the upstream lip is also designed so that it steps toward the web with respect to the middle lip. This also achieves an increasing pressure gradient in the upstream direction and assists in sealing the bead under the die lips to avoid upstream leakage. There is always recirculation in the bottom layer under the upstream lip. However, typically, such recirculation is open so that it does not negatively affect the quality of the bottom layer. This upstream lip can be xe2x80x9cflatxe2x80x9d or parallel to the web, or it may be beveled or angled with respect thereto. Preferably, the bevel represents a divergence in the sense of the web travel. This profile presents a positive pressure gradient in the upstream direction, which further assists in sealing the bead.
When the upstream and downstream lips of the present method are beveled, the middle lip is preferably maintained close to flat (in the sense that it is approximately parallel to the web, not taking into consideration any curvature). This can be achieved, even during operation, since angle of attack adjustments are minimized due to the beveling of the aforementioned lips. The flatness of the middle lip, together with an appropriate coating gap, provides a zero pressure gradient to the flow, which advantageously avoids recirculations and still reduces shear rate and shear stresses, as discussed above. A flat middle lip also has the advantage of reducing the risk of upstream leakage. Moreover, this middle lip is the most expensive to manufacture, and the absence of a bevel assists in reducing costs.
It should be noted that other lip geometries are possible in order to achieve the advantages of the present invention. Also, other methods of pressure regulation are possible.
In another aspect of the present invention, pressure gradient regulation can also be achieved with lip designs of a particular length, especially that of the middle and downstream lips. That is, it will be appreciated that the length of the die lips will affect the coating gap if the angle of attack of the die is adjusted. Typically, with a negative angle of attack (a convergence of the die lips with the web in the downstream direction), the coating gap at the upstream portion of each lip is greater than at the downstream portion of each lip. This is especially true, considering the curvature of the back-up roll. As noted above, if coating gaps are too great, recirculations will occur due to inappropriate pressure gradients, thus causing the loss of control of separating line position and poor coating quality.
In addition, as noted above, the flow experiences shear stresses in the bead due primarily to the rapidly moving web. Even if the shear rate is tolerable with respect to fluid properties, the duration of the shear can have damaging effects on liquid quality. The longer the lips, the greater the duration of the shear stresses experienced by the liquid. Thus, it is important when designing the die lip geometries, to consider the length of the die lips for coating gap, as well as shear stress considerations.
Therefore, it is an important aspect of the present method that the lip lengths are minimized, while providing sufficient length to develop stable rectilinear flow. Perhaps the most important die lip length is the downstream lip. This lip must be long enough for the flow to develop. Such lip may be in the range of 0.1-3.0 mm in length, with about 0.8-1.2 mm being preferable. The middle lip also may range from 0.1-3.0 mm, but is preferably about 0.3-0.7 mm in length. The upper lip, on the other hand, can be longer without suffering shear stresses in the liquid because the length of travel is reduced. Moreover, a longer upstream lip assists in sealing the bead. Thus, a lip in the range of 1.0-3.0 mm is advantageous, with 1.5-2.5 mm being preferable.
Thus, the present method of multilayer coating has a downstream feed gap region characterized by a pressure gradient which generates stable flow at the interface between a bottom layer (including any previously coated layers) and a top layer. For the embodiments described above, this pressure gradient is achieved by a combination of middle lip and downstream lip geometries, which result in an adequate pressure gradient at the interface region which is not so positive as to cause recirculations.
In addition to the correct design of the die lip geometries and the assembly and setup of the die with respect to the web so that correct coating gaps are achieved, the present method also involves a careful analysis of certain fluid parameters with respect to the liquids to be coated on the web. In particular, the present method involves an analysis of the relative viscosities of the two liquids. Preferably, the viscosity of the top layer liquid should be greater than the viscosity of the bottom layer liquid. More specifically, a top layer viscosity which is about 30% greater than the bottom layer viscosity is optimal; however, successful multilayer coating can be achieved when the top layer viscosity ranges from about 50% less to 100% (or even more) more than the viscosity of the bottom layer. However, it will be recognized by those of ordinary skill that these ranges may vary even outside of these boundaries for a given set of coating parameters.
This balancing of viscosities is important in order to assist the process in achieving steady, two-dimensional flow. However, because the flow experiences such high shear rates, the viscosity analysis must take into consideration the change in viscosity due to such shear rates. Thus, for example, due to shear thinning, the viscosity of any liquid being coated may vary by several orders of magnitude of milliPascal-seconds (mPa-sec). At the same time, the shear rate may vary by four or more orders of magnitude with respect to the film coating parameters involved with the present method. In particular, shear rates above 1,000 sxe2x88x921 are likely to be experienced under such coating conditions. Accordingly, the relative viscosities of the liquids being coated should be compared at these higher shear rates.
In addition, the surface tensions of the respective liquids should be analyzed, with the top liquid preferably having a lower surface tension than the bottom liquid. This condition helps to avoid the formation of voids in the top layer with respect to the bottom layer which may be formed by de-wetting phenomena.
Once the lip geometries have been designed and set with respect to the die, and the liquid parameters analyzed, another important aspect of the present invention is the experimental determination of the area of operating parameters in which successful coating can be achieved. This area is often referred to as the xe2x80x9ccoating windowxe2x80x9d and may be defined in terms of a graph of coating gap versus angle of attack of the die. Thus, in order to determine a coating window, samples of the two liquids are experimentally coated at varying coating gaps and angles of attack and the coating quality is observed. The area where adequate coating is achieved is noted, including the area where very high quality coating is achieved (usually a subset of the overall coating window). It is preferable that the coating window be as large as possible so that inaccuracies in coating gap and/or angle of attack do not result in coating defects or product degradation. In order to add another dimension to the coating window, the same liquids being tested are also tested at various viscosities.
Once the coating window is determined, production coating may occur preferably at a point in the middle of the range of the angles of attack and close to the maximum coating gap and angle of attack.
When a dual die is used to simultaneously coat a support layer and release layer the resulting multilayer release surface has several desirable features. First, because the support layer and silicone layer are coated substantially simultaneously as liquids, the interface between the support layer and silicone layer is not as sharp and distinct as if the support layer had been cured or hardened prior to the coating of the silicone layer. This is beneficial for certain applications, because the increased dispersion observed between the layers facilitates binding of the silicone layer to the support layer, and therefore decreases the propensity of the silicone layer to rub-off or otherwise separate from the support layer. Second, because the coating parameters of the support layer and silicone layer are so tightly controlled by the present method, the degree of dispersion of the two layers is minimized to substantially the extent necessary to achieve desirable bonding between the support layer and the silicone layer, without undue waste of dispersed silicone in the support layer. Finally, dual die coating may be used to form a multilayer release surface from a support layer and release layer which would not form a stable curtain for curtain coating because the surface energies differ by too much.
The improved dispersion characteristics of the supporting and silicone layers comprising the multilayer release surfaces of the present invention can be characterized in several ways. One preferred way is by transmission electron microscopy (TEM). When TEM is applied to multilayer release systems of the prior art and the dual die constructs of the present invention, it is observed that two distinct layers, comprising the support layer and the silicone release layer, are formed from both processes. However, the borders of the layers of the prior art coatings are much sharper, indicating that there is minimal intermixing of the support layer and the silicone release layer. In contrast, TEM scans of multilayer release constructs of the present invention show that, while having well defined borders, there are a small amount of silicone domains in the support layer, which is indicative of desirable bonding within the layers.
In summary, the method of the present invention enhances the optimization of the coating process for multilayer release surfaces. The method can be utilized with a wide variety of coatings and substrates in order to produce multilayer release surfaces on open paper surfaces which have release properties equal to or better than those produced on high quality papers.